Hippolyte Bayard was not the only one to experiment with a direct positive process, particularly after the advent of the Daguerreotype. Fox Talbot thought that this may be the way to go, thus not realising at the time the full advantage of his own replication process. I would suggest from my own research in this area, that fogging with the first halogen was followed by expose with a second. This is a very interesting area for practical research and one which promises many possibilities for new forms of chemical imagery.